The present invention relates to an output circuit for outputting a control signal to an apparatus to be controlled in accordance with the control signal.
FIGS. 8 and 9 are block diagram showing conventional output circuits. In FIG. 8, the output circuit, designated at numeral 1100 is of a source output type comprising an internal circuit 1102 for outputting a control result and a photo-coupler 1104. A controlled apparatus 1106 to be coupled to the output circuit 1100 is equipped with an external power supply 1108 and an output detecting section 1109. Further, the photo-coupler 1104 comprises an LED 1104A and an output transistor 1104B. With this arrangement, when the photo-coupler 1104 takes an ON state, a current flows through the output detecting section 1109 whereby the operating state of the internal circuit 1102 is detectable at the controlled circuit 1106 side. On the other hand, an output circuit 1200 illustrated in FIG. 9 is of a sink output type comprising an internal circuit 1202 for outputting a control result and a photo-coupler 1204. A controlled apparatus 1206 to be coupled to the output circuit 1200 includes an external power supply 1208 and an output detecting section 1209. The photo-coupler 1204 comprises an LED 1204A and an output transistor 1204B.
There is a problem which arises with the conventional output circuit 1100 or 1200, however, in that, when coupling the output circuit 1100 or 1200 to the controlled apparatus 1106 or 1206, or when turning on the external power supply 1108 or 1208, the output transistor 1104B or 1204B momentarily takes the ON state so that an error output occurs. Thus, there is the possibility that the controlled apparatus 1106 or 1206 malfunctions. This is because, when a rapidly rising voltage is applied to between the collector and emitter of the output transistor 1104B or 1204B, a current passes through a junction capacity portion C1 between the collector and base of the transistor 1104B or 1204B (such as illustrated in FIG. 9) and hence the output transistor 1104B or 1204B instantaneously takes the ON state. In other words, this is due to the so-called flash-on phenomenon.